Low temperature curing nylon-epoxy-
phenolic adhesive

ABSTRACT

THIS PATENT DISCRIBES A NOVEL ADHESIVE COMPOSITION COMPRISING AN INIMATE HOMOGENEOUS MIXTURE OF A NYLON WHICH IS A POLYAMIDE FORMED FROM THE REACTION HEXAMETHYLENE DIAMMONIUM ADIPATE AND HEXAMETHYLENE DIAMMONIUM SEBACATE WITH CAPROLACTAM, AN EPOXY RESIN WHICH IS A POLYGLYCIDYL ETHER OF A POLYHYDRIC PHENOL, AND A PHENOLIC RESIN WHICH IS AT LEAST DIFUNCTIONAL IN PHENOLIC HYDROXYL GROUPS PRIOR TO CURING, SAID MIXTURE BEING IN THE FORM OF A FILM OF SUBSTANTIALLY UNIFORM THICKNESS AND WEIGHT PER UNIT AREA.

United States Patent 27,426 LOW TEMPERATURE CURING NYLON-EPOXY- PHENOLIC ADHESIVE James David Lincoln, Costa Mesa, and J olm F. Wasmund,

San Clemente, Calif., assignors to Whittaker Corporation, Los Angeles, Calif.

No Drawing. Original No. 3,496,248, dated Feb. 17, 1970,

Ser. No. $73,463, Aug. 19, 1966. Application for reissue Oct. 8, 1970, Ser. No. 79,303

Int. Cl. C08g 45/08 US. Cl. 260-831 13 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE In the past, high peel strength structural adhesives contained a blend of a very high molecular weight flexible polymer with a low melting thermosetting resin. Examples of such blends include vinyl-phenolic rubber, rubber phenolic and more recently nylon-epoxy resins. The high molecular weight, flexible portion of the blends contributed toughness and peel strength while the thermosetting component provided creep resistance and specific solvent resistance.

However, these materials possess several disadvantages.

Thus, in order to make satisfactory joints, it was neces- V sary to cure these materials at a temperature of 300 F. to 350 F. using pressures of 40 to 100 p.s.i., since high cure temperatures and pressures are necessary in order to cause the high molecular weight portion of these blends to flow and wet the adherends. High curing temperatures and pressures, though tolerable, cause many difficulties including weakening of aluminum alloys, high residual thermal stresses in complex structures, and high processing cost because of the expensive tooling fixtures and equipment required to bond structures at high temperature and pressure.

(In contrast to the foregoing, the present invention provides a family of high peel strength structural adhesives which can be bonded at temperatures as low as 180 F. The novel structural adhesives of the present invention therefore will substantially avoid the various difficulties which have been observed to be present with the abovediscussed adhesives of the prior art.

Thus, it is an object of this invention to provide a novel class of low temperature curing, high peel strength structural adhesives.

Another major object of the invention is to provide a method of obtaining a new low temperature curing, high peel strength structural adhesive.

More specifically, it is an object of the invention to provide novel epoxy-n'ylon-phenolic adhesives, as are more fully hereinafter described.

In a more analytic sense, it is an object of the invention to utilize the hydroxyl content of the phenolic resins as a thermal setting solvent for nylon epoxy adhesive sys tems.

These and other objects and advantages of this invention will be apparent from the more detailed description which follows.

Briefly, the present invention compriss a family of adhesive formulations containing nylon, epoxy resins, and phenolic resins. More particularly, the present invention comprises a novel adhesive formulation ordinarily containing from 5 to about 65 parts by weight nylon, from about 25 to about 70 parts by weight epoxy resin, and from 5 to about 35 parts by weight of phenolic resins, which are at least difunctional in phenolic hydroxyl groups. The present invention additionally includes the preparation of these novel adhesive formulations.

In this invention, it has been observed that the high molecular weight nylon contributes toughness while the epoxy-phenolic portion provides creep resistance and specitic solvent resistance. While both nylon-epoxy and phenolic-epoxy combinations are common in the prior art, the combination of nylon-epoxy with phenolic resins is novel, and unexpectedly yields a structural adhesive which permits a significant lowering of cure temperatures due to the fact that the melt temperature, sometimes referred to as the wetting temperature, of the nylon is lowered by the presence of the phenolic-hydroxyl groups present in the phenolic resins. While not bound by any theory, it is believed that the phenolic resin is, in fact, a solvent for the nylon.

In general, it has been previously observed that solvents tend to degrade the properties of adhesive bonds. Thus, if a solvent is used to enhance wetting or lower melt temperature, it normally must be removed or else a weak non-structural joint will occur. It is believed that in the case of the novel nylon-epoxy-phenolic adhesives of the present invention, the phenolic solvent is removed after wetting occurs by copolymerization with the epoxy portion of the blend. Thus, the phenolic portion of the adhesives could be considered to be a thermal setting solvent.

The novel adhesives of the present invention are particularly adapted for both metal-to-metal and sandwich applications where high strength at temperatures from about 67 F. to F. is required. It has been found that the low temperature cure of 250 F. minimizes corrosion of the aluminum parts, and prevents degradation of plastic and plastic tooling which normally occurs where high curing temperatures are required. Moreover, in the use of adhesives of the present invention, the cure temperature is not only lower but the curing time is reduced to almost half of that required by the higher curing adhesives of the prior art. Adhesives of the present invention provide still other savings by reducing autoclave times, lower processing times, and lower tooling costs. It has been found that the adhesives of the present invention provide excellent handleability and drapability for both metal-to-rnetal and sandwich applications. Typical applications for the novel adhesives of the present invention include radar transit cases, cryogenic bulkheads, helicopter rotor blades, Wing and enpennage components, electrical components, wing tip bonding, speed brake doors, leading edges, trailing edges, and wing paneling.

In general, any of the phenol-aldheyde resins having at least two free phenolic hydroxyl groups in the uncured state may be combined with epoxy resins and nylon to form the novel adhesive material within the scope of the present invention. Thus, the phenol-aldehyde resins used in the present invention may be prepared from various phenols such as phenol, cresol, cathecol, xylenol, hydroquinone, resorcinol, phenyl phenol o-r butyl phenol, and various aldehydes, such as formaldehyde, and furfuraldehyde. The resins prepared primarily from phenol and formaldehyde are preferred. The phenol-aldehyde were stored in a refrigerator in a polyfilm bag. Several days later, the adhesive films in the polyfilm bags were removed from the refrigerator and permitted to come to room temperature. Thereafter, laynps were prepared from the adhesives in the manner above-described. The cure temperature was 250 and the cure pressure 40 psi. for ninety minutes. The mechanical properties of the joined adherends employing the adhesive of this example are set forth in the following table:

solvent containing said nylon, epoxy resin and phenolic resin on a base, and permitting the solvent to evaporate.

5. A method of forming a fused adhesive film, the steps comprising admixing in a solvent an ethyl alcohol/Watermixture soluble nylon which is a polyamide formed from the reaction of hexamethylene diammonium adipate and hexamethylene diammonium sebacate with caprolactam, an epoxy resin which is a polyglycidyl ether of a polyhydric phenol, and a phenolic aldehyde resin which is at TABLE Test Results mp. Property Unit C F.) No prime Prime Tensile shear strength. RT 6, 500 6, 700 Tensile shear strength- 180 3, 500 3, 800 Tensile shear strength 250 1, 200 1,400 Tensile shear strength 67 5, 200 4, 900 Tensile shear strength:

30-days salt spray P.s.i RT 3, 900 4, 400 EEO-days in tap water P.s.i RT 5, 600 5, 900 7-days in ant-icing fluid P .51. RT 6, 250 6, 100

' 7-days in hydraulic oil Psi RT 6, 750 6, 450

(Mil-F-5606). 7-d2ys in JP-4 iuel (Mil-O- P.s.i RT 6, 800 6, 650

5 24 7-days in hydrocarbon fluid. P.s.i RT 6, 200 6, 000 -days condensing humidity P.s.i RT 3, 600 4, 100

100% REL at 120 F. Metal-to-metal T peel Lb./in RT 45 45 (0.020) 2024T-3 clad alum. Sandwich peel strength (Mil- In. lb./in. width... RT 45 45 A-25463 substrates).

It will be understood that many variations are possible within the scope of the present invention. Accordingly, it is intended that the invention be limited only by the lawful scope of the appended claims.

What is claimed is:

1. An adhesive composition comprising an intimate homogeneous mixture of an ethyl alcohol/water-mixture soluble nylon which is a polyamide formed from the reaction of hcxamethylene diammonium adipate and hexamethylene diammonium sebacate with caprolactam, an epoxy resin which is a polyglycidyl ether of a polyhydric phenol, and a phenolic aldehyde resin which is at least difunctional in phenolic hydroxyl groups prior to curing, said composition containing from 5 to about 65 parts by weight nylon, from about 25 to about 70 parts by weight epoxy and from 5 to about parts by weight of aldehyde phenolic resin and said mixture being in the form of a film of substantially uniform thickness and Weight per unit area.

2. The adhesive composition of claim 1 in which said mixture includes resorcinol.

3. An adhesive material comprising an intimate homogeneous mixture of an ethyl alcohol/water-mixture soluble nylon which is a polyamide formed from the reaction of hexamethylene diammonium adipate and hexamethylene diammonium sebacate with caprolactam, a curable epoxy resin which is a polyglycidyl ether of a polyhydric phenol, and a curable phenolic aldehyde resin which is at least difunctional in phenolic hydroxyl groups prior to curing, said composition containing from 5 to about '65 parts by weight nylon, from about 25 to about 70 parts by weight epoxy and from 5 to about 35 parts by weight of said phenolic resin.

4. The method of forming an adhesive which comprises the steps of admixing in a solvent an ethyl alcohol/ water-mixture soluble nylon which is a polyamide formed from the reaction of hexamethylene diammonium adipate and hexamethylene diammonium sebacate with caprolactam, an epoxy resin which is a polyglycidyl ether of a polyhydric phenol, and a phenolic aldehyde resin which is at least difunctional in phenolic hydroxyl groups prior to curing said composition containing from 5 to about 65 parts by weight nylon, from about 25 to about 70 parts by weight epoxy and from 5 to about 35 parts by Weight of said phenolic resin and thereafter coating said least difunctional in phenolic hydroxyl groups prior to curing said composition containing from 5 to about 65 parts by weight nylon, from about 25 to about 70 parts by weight epoxy and from 5 to about 35 parts by weight of said phenolic resin and coating said solution on a base, permitting said solvent to evaporate, and subjecting said resulting film to heat and pressure to yield a fused adhesive.

6. The combination of claim 1 wherein the phenolic resin is a phenol-formaldehyde resin.

7. The composition of claim 1 wherein the epoxy resin is a polyepoxide from the condensation of epichlorohydrins with polyhydric phenols.

8. A composite structure comprising two metal parts joined by adhesive composition comprising an intimate homogeneous mixture of an ethyl alcohol/water-mixture soluble nylon which is a polyamide formed from the reaction of hexamethylene diammonium adipate and hexamethylene diammonium sebacate with a caprolactam, an epoxy resin which is a polyglycidyl ether of a polyhydric phenol, and a phenolic aldehyde resin which is at least difunctional in phenolic hydroxyl groups prior to curing, said composition containing from 5 to about 65 parts by weight nylon, from about 25 to about 70 parts by weight epoxy and from 5 to about 35 parts by weight of said phenolic resin and said mixture being in the form of a film of substantially uniform thickness and weight per unit area.

9. The method of claim 4 wherein the phenolic resin is a phenol-formaldehyde resin.

10. The method of claim 4 wherein the epoxy resin is a polyepoxide from the condensation of epichlorohydrins with polyhydric phenols.

11. An adhesive composition comprising an intimate homogeneous mixture of (a) a curable epoxy resin which is a polyglycidyl ether of a polyhydric phenol, (b) a curable phenolic-aldehyde resin which is at least difunctional in phenolic hydroxyl groups prior to curing, and (c) a polyamide having repeating carbonamide groups as an integral part of the linear polymeric chain and which is soluble in said curable phenolic-aldehyde resin, said composition containing from about 25 to about 70 parts by weight of (a), about 5 to about 35 parts by weight of (b), and about 5 to about 65 parts by weight (0).

12 The adhesive composition of claim 11 wherein said UNITED STATES PATENTS ggtlfle phenolic-aldehyde resm 1s a phenol-formaldehyde 9 9 4 rd n 2 1 J3 The adhesive comp'osz'tiohof claim 12 wherein said 3 44 1 9 7 Fh irable epoxy resin is a polyepoxide from the condensa- 5 3 02 6 E i 1 Q on of ep'ichlorohydrins with polyhydric phenols. 3,379,561 4/1968 Manaka 260-831 References Cited PAUL UEBERMAN, Primary Examiner The following references, cited by the Examiner, are E record in the patented file of this patent or the original 10 US itent. 26047 EP, 830 P 

